This invention relates to LC responders and particularly to LC responders that are placed within objects such as toys. In operation, an interrogation or base unit sends a pulse and passive responders reply with their unique frequency which is sensed by the base unit triggering a particular response such as turning on a motor etc.
The prior art includes a technology used for anti-theft retail security tags. Specific prior art patents range more broadly and include U.S. Pat. Nos. 4,002,263 and 4,991,350 which relate to magnetically actuated pet doors. While both patents relate to bi-directional pet doors which appear to use retaining magnets, neither discloses the use of a frequency selective key.
The prior art also includes U.S. Pat. Nos. 3,863,244, 3,868,669, 3,919,704, 4,281,321 and 4,369,557 which are merely examples of alarm devices using passive responders or the like.
The toy business requires extremely inexpensive circuits which are simple and reliable. Some of the problems solved by this invention involve the recognition of frequencies at a distance, the orientation of the transmitter receiver to the responder and the receiving of simultaneous signals from more than one responder. The prior art devices are generally designed to accept or reject a specific frequency.